Computed tomography (CT) is a widespread diagnostic imaging method which measures, in its imaging process, the radiodensity (electron density) of matter. This radiodensity is depicted using CT in terms of Hounsefield Units (HU). Hounsefiled Units, named after the inventor of the first CT scanner, reflect the relative absorption of CT X-rays by matter, the absorption being directly proportional to the electron density of that matter. Water, for example, has a value of 0 HU, air a value of -1000 HU, and dense cortical bone a value of +1000 HU. Because of the similarity in density of various tissues in the body, however, contrast agents have been sought to change the relative density of different tissues, and improve the overall diagnostic efficacy of this imaging method.
In the search for contrast agents for CT, researchers have generally sought to develop agents that will increase electron density in certain areas of a region of the body (positive contrast agents). Barium and iodine compounds, for example, have been developed for this purpose. For the gastrointestinal tract, barium sulfate is used extensively to increase the radiodensity of the bowel lumen on CT scans. Iodinated water soluble contrast media are also used to increase density within the gastrointestinal tract, but are not used as commonly as the barium compounds, primarily because the iodine preparations are more expensive than barium and prove less effective in increasing radiodensity within this region of the body.
Despite their widespread use, however, barium and iodine compounds are suboptimally effective as gastrointestinal contrast agents for CT. For example, if the concentration is too low, there is little contrast. Conversely, if the concentration is too high, then these radiodense contrast agents cause beam hardening artifacts which are seen as streaks on the CT images. It is also difficult to visualize the bowel mucosa with either the barium or iodine contrast agents.
In an attempt to improve upon the efficacy of contrast agents for the gastrointestinal tract, lipid emulsions that are capable of decreasing electron density (negative contrast agents) have been developed. Because lipids have a lower electron density than water, lipids provide a negative density on CT (a negative HU value). While these lipid emulsions appear to be more effective than the barium and iodine agents at improving visualization of the mucosa of the bowel, these contrast agents have limitations. First, there is a limitation to the concentration of lipid which a patient can tolerably drink, which puts a limit on the change in density (or HU) which the lipid based CT contrast agent can provide. Lipid emulsions are also frequently expensive. Furthermore, these lipid formulations are generally perishable, which provides for packaging and storage problems.
New and/or better contrast agents for computed tomography imaging are needed. The present invention is directed toward this important end.